[media] bttv/cx25821/cx88/ivtv: use sg_next instead of sg++

Never use sg++, always use sg = sg_next(sg). Scatterlist entries can
be combined if the memory is contiguous but sg++ won't know about that.

As far as I can tell cx88 and ivtv are really broken because of this,
and bttv and cx25821 are OK because vb1 doesn't combine scatterlist
entries.

But regardless, sg++ should never be used, only sg_next is safe.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>